Field of the Invention
The invention is related to a growth method of group III nitride crystals in supercritical ammonia and the group III nitride crystals grown by the method. A high-pressure reactor is used to grow bulk crystal of group III nitride in supercritical ammonia. Group III nitride crystals are used to produce semiconductor wafers for various devices including optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs), and electronic devices such as transistors. More specifically, the group III nitride includes gallium.
Description of the Existing Technology
(Note: This patent application refers several publications and patents as indicated with numbers within brackets, e.g., [x]. A list of these publications and patents can be found in the section entitled “References.”)
Gallium nitride (GaN) and its related group III nitride alloys are the key material for various optoelectronic and electronic devices such as LEDs, LDs, microwave power transistors, and solar-blind photo detectors. Currently LEDs are widely used in displays, indicators, general illuminations, and LDs are used in data storage disk drives. However, the majority of these devices are grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide because GaN substrates are expensive compared to these heteroepitaxial substrates. The heteroepitaxial growth of group III nitride causes highly defected or even cracked films, which hinder the realization of high-end optical and electronic devices, such as high-brightness LEDs for general lighting or high-power microwave transistors.
To solve all fundamental problems caused by heteroepitaxy, it is indispensable to utilize crystalline group III nitride wafers sliced from bulk group III nitride crystal ingots. For the majority of devices, crystalline GaN wafers are favorable because it is relatively easy to control the conductivity of the wafer and GaN wafer will provide the smallest lattice/thermal mismatch with device layers. However, due to the high melting point and high nitrogen vapor pressure at elevated temperature, it has been difficult to grow GaN crystal ingots. Currently, the majority of commercially available GaN substrates are produced by a method called hydride vapor phase epitaxy (HVPE). HVPE is a vapor phase method, which has a difficulty in reducing dislocation density less than 105 cm−2.
To obtain high-quality GaN substrates of which dislocation density is less than 105 cm−2, a method called ammonothermal growth has been developed [1-6]. Recently, high-quality GaN substrates having dislocation density less than 105 cm−2 can be obtained by the ammonothermal growth. The high-pressure reactor of ammonothermal growth must be constructed with Ni—Cr based superalloy due to extreme temperature and pressure conditions. The maximum diameter of a Ni—Cr superalloy reactor is limited by the material of construction, its properties, the high pressure and temperature, and the chemical aggressiveness of the chemicals within the alloy. The chamber diameter of the high-pressure reactor is therefore limited to a rather small maximum value. In order to increase the number of crystals grown simultaneously in one reactor, the reactor length must be extended. However, if the reactor length is extended, the distance between the nutrient and the seed crystal farthest from the nutrient will become too large for crystal growth to occur, since the reactor is a closed or batch reactor due to reactor conditions and limitations on material of construction and since chemical transport is by natural convection within the reactor.